Electrophotographic photoreceptors using an organic photoconductive substance have been extensively studied because of advantages such as no pollution, high productivity and low production cost. An electrophotographic photoreceptor containing a diphenyldicyanoethylene derivative as a sensitizer in a light-sensitive layer thereof is known as described in, for example, JP-A-54-30834 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
An organic photoconductive substance which generates an electric charge upon absorption of visible light is poor in electric charge-retaining force. Conversely, an organic photoconductive substance which is good in electric charge-retaining force and excellent in film-forming properties generally does not exhibit any substantial photoconductivity upon application of visible light. In order to overcome the above problem, a laminate type light-sensitive layer has been designed, i.e., a layer structure that is functionally separated into an electric charge-generating agent which generates an electric charge upon absorption of visible light and an electric charge-transporting agent which transports an electric charge. A number of electric charge-generating agents and electric charge-transporting agents have been proposed. Known positive hole-transporting agents include amine compounds, hydrazone compounds, pyrazoline compounds, oxazole compounds, oxadiazole compounds, stilbene compounds, carbazole compounds and the like, and known electron-transporting agents include 2,4,7-trinitrofluorenone and the like. In addition, boron-containing compounds are described, for example, as a photoconductive substance in JP-B-48-9988 (the term "JP-B" as used herein means an "examined Japanese patent publication"), and as a fluorescent agent in Canadian Patent No. 912,019.
A sensitizer, which is sufficient for use with an electrophotographic photoreceptor of a single layer structure using an organic photoconductive substance has not been known in the art. A positive charging type material is desirable for use in a function separation type electrophotographic photoreceptor of the laminate structure for preventing generation of ozone in corotron and for controlling charging of a toner in development. In the case of the positive charging type material, when the electric charge-transporting agent is positive hole-transporting, it is necessary for the electric charge-generating layer to be provided as an upper layer. The electric charge-generating layer is usually made thin in view of its function and fails to sufficiently satisfy mechanical characteristics as a photoreceptor. Moreover, some modifications should be made to a copying machine, to use it in a negative charging system. Thus a positive charging type photoreceptor having a relatively thick electric charge-transporting layer as an upper layer is desired, and in this case, it is necessary to use an electron-transporting electric charge-transporting agent in the electric charge-transporting layer. However, of electron-transporting electric charge-transporting agents conventionally proposed, no sufficiently satisfactory substance has been known.